Journal Article

Chirality-assisted lateral momentum transfer for bidirectional enantioselective separation

Zhang Tianhang, Ding Weiqiang, Shi Yuzhi, Sáenz, Juan José, Liu, Ai Qun, Zhu Tongtong, Cipparrone Gabriella, Cheng-Wei, Qiu, Tsai, Din Ping, Mazzulla Alfredo

2020

Overview

Lateral optical forces induced by linearly polarized laser beams have been predicted to deflect dipolar particles with opposite chiralities toward opposite transversal directions. These “chirality-dependent” forces can offer new possibilities for passive all-optical enantioselective sorting of chiral particles, which is essential to the nanoscience and drug industries. However, previous chiral sorting experiments focused on large particles with diameters in the geometrical-optics regime. Here, we demonstrate, for the first time, the robust sorting of Mie (size ~ wavelength) chiral particles with different handedness at an air–water interface using optical lateral forces induced by a single linearly polarized laser beam. The nontrivial physical interactions underlying these chirality-dependent forces distinctly differ from those predicted for dipolar or geometrical-optics particles. The lateral forces emerge from a complex interplay between the light polarization, lateral momentum enhancement, and out-of-plane light refraction at the particle-water interface. The sign of the lateral force could be reversed by changing the particle size, incident angle, and polarization of the obliquely incident light.Optical sorting: Forcing mirror-image microparticles to go their own wayA setup that uses lasers to automatically sort compounds with nearly imperceptible chemical differences could improve the efficiency of drug manufacturing. Many pharmaceuticals are difficult to purify because they are chiral, meaning they exist as one of two mirror-image structures that rotate in opposite directions under linearly polarized light. Cheng-Wei Qiu from the National University of Singapore and colleagues now report that lateral forces generated by a mix of reflected and refracted light rays can separate chiral microspheres floating on the surface of water. The team developed a line-shaped optical trap to hold a mix of chiral particles in place, and then directed a polarized laser beam into the trap. Microscopic imaging revealed that particles with identical chirality moved laterally when exposed to the beam, with velocities dependent on incident angle and microparticle size.

• Publisher: Springer Nature B.V
• Subject: Chirality / Enantiomers / Handedness / Lasers / Microparticles / Microspheres / Optics / Polarization / Polarized light